Polyvinylchloride resins, having excellent flame retardance, have been generally used as a coating material for coated electric wires employed to parts of automobiles, electric and electronic devices, and the like.
However, the polyvinylchloride resins, contrary to their excellent flame retardance, have a problem that, since they contain halogen elements in their molecular chains, they release harmful halogenous gases to atmosphere while the automobiles are fired or the electric and electronic devices are burned for disposal, causing environmental pollution.
To solve this problem, flame-retardant resin compositions freed from halogens are developed which comprise polyethylene or polypropylene as a base resin and a metal hydrate such as magnesium hydroxide as a flame retardant. These flame-retardant halogen-free resin compositions, however, have a disadvantage of poor mechanical properties such as abrasion resistance, since they require addition of metal hydrates in a large amount as the flame retardant.
To solve such disadvantage mentioned above, a coated electric wire having two coating layers has been developed (Patent Document 1).
The coating layers of the coated electric wire are composed of an outer layer and inner layer with blending a flame retardant in the inner layer in larger amount than in the outer layer, thereby preventing the outer layer from deterioration of mechanical properties as well as retaining more flame retardancy in the inner layer.
However, if the coated electric wire relies only on an inner layer about its flame retardancy, a large amount of flame retardant needs to be added, and the inner layer drastically loses the flexibility thereof, deteriorating fundamental properties required to the electric wire. Therefore, two-layer coated electric wires conventionally provided cannot avoid to add a certain amount of a flame retardant in the outer layer thereof. Other additives, such as antiaging agents, are also added to the outer layer for various purposes. However, such additives are added in so large amount that resulting mechanical properties such as abrasion resistance and scratch damage resistance are not necessarily satisfactory.
Moreover, requirements of weight saving for various coated electric wires currently have become stronger. Particularly, according to requirements of lightening a body of automobiles, the requirements of saving weights of coated electric wires spread not only to reducing diameter of conductors but also to thinning coating layers. As the result, scratch damage resistance of the coated electric wires has become a big issue. In other words, when thick coating layers are allowed, the problems of scratch damage don't appear, but when the thickness of the coating layers becomes smaller, conductors could be exposed by even a slight scratch damage.
Furthermore, as wiring becomes more complex, flexibility of coated electric wires becomes more important, causing difficulty to add flame retardants to the inner layer in a large amount.
Patent Document: JP 1-302611A.